Battelle Ingenieurtechnik (Germany)

A 21 kHz standing-wave amplifier at the focus of a 36 cm diameter parabolic mirror (ka=70) is used for highly directive radiation of low frequency sine waves, voice signals and even music (of poor quality). The low frequency waves are amplitude-modulated onto the ultrasonic carrier wave and radiated into a solid angle of ±1° (−3 dB). Low frequency sound waves with frequencies between 50 Hz and 3 kHz are radiated over a distance of more than 20 m. The achievable audio sound pressure levels decrease with approximately 20 dB per distance doubling and reached about 30 dB at 20 m distance. The discussion of the measurements is based on Blackstock’s weak shock theory [5] and on the nonsaturated radiation pressure at saturated sound pressure levels.